This book provides a comprehensive overview of recent developments in the fast moving field of protein transport across and into intracellular membranes. The soluble and membrane-bound components assisting in these processes are introduced, and their functions described. Besides a detailed analysis of protein translocation across the bacterial plasma membrane, the mechanisms of protein targeting within the eukaryotic cell are discussed with special focus on the transport of proteins to the endoplasmic reticulum, vacuoles, peroxisomes, mitochondria, and chloroplasts. Providing stimulating hypotheses and models, the book is well suited for advanced students, graduate students, and for newcomers to the field seeking a general, yet accurate introduction to present knowledge in membrane transport of proteins.